1. Field of the invention
The invention relates to an optical add/drop multiplexer, and more specifically, to a reconfigurable optical add/drop multiplexer (ROADM).
2. Description of the Related Art
In the optical network, an optical add/drop multiplexer (OADM) is used to drop an optical signal with a specific wavelength from a multi-wavelength optical signal, and to add another optical signal with the same wavelength to the multi-wavelength optical signal. For example, the optical add/drop multiplexer may drop an optical signal with a specific wavelength from a backbone of an optical fiber network to a local area network (LAN), and may add another optical signal with the same wavelength from the LAN to the backbone. Consequently, the signal transmission architecture of the overall optical fiber network is more flexible.
Among the optical add/drop multiplexers, a reconfigurable optical add/drop multiplexer (ROADM) may decide whether or not to add and drop optical signals according to an external instruction, and may tune the wavelengths of the to-be-added and to-be-dropped optical signals. In the prior art, a reconfigurable optical add/drop multiplexer receives a multi-wavelength optical signal, and then divides the received multi-wavelength optical signal into multiple optical signals having multiple specific wavelengths. Subsequently, optical signals with different wavelengths are transmitted in different waveguide elements which are connected to a 2×2 optical switch that is used to switch the to-be-dropped optical signal. However, the drawback of this kind of tunable reconfigurable optical add/drop multiplexer resides in its complicated structure. Thus, multiple filters and optical switches have to be used in order to drop the optical signals with specific wavelengths, thereby increasing the cost and the difficulties of assembly.
In another tunable ROADM according to the prior art, multiple Bragg gratings are used to drop optical signals with different specific wavelengths. By switching between the Bragg gratings having different periods, the tunable ROADM may tune the to-be-dropped and to-be-added wavelengths. However, in addition to the optical switches, the tunable ROADM still has to use an optical circulator to separate the optical signals inputted to the Bragg gratings from the optical signals reflected from the Bragg gratings. Therefore, this method does not reduce the cost or the difficulties of assembly.